1. Field of the Invention
The instant disclosure relates to a DC motor control method; in particular, to a DC motor control method and a DC motor control circuit.
2. Description of Related Art
The DC motor has been widely used in many electronic products, such as personal computers, electric shavers, copiers, projectors, blenders and other kinds of low voltage electronic products. With the advancement of technology, demand for personal computers with high performance has also increased continuously. For a personal computer with high performance, temperature of a host rises easily because of the increase of the operation speed of the central processing unit in the unit time. Therefore, the new generation of personal computers needs to use fans with DC motors for cooling, and thus DC motor fans play a decisive role in the miniature heat-dissipation fan system. For the above reasons, it is important to provide a circuit and control method thereof for stabilizing operation of the DC motor.
Referring to FIGS. 1 and 2, FIG. 1 shows a circuit block schematic diagram of the control circuit of the single-phase DC motor in the prior art. FIG. 2 shows a waveform schematic diagram of the output signal in FIG. 1. The control circuit of the single-phase DC motor senses a magnetic pole position of an inner rotator of the single-phase DC motor 130 through utilizing a hall element HAL, and accordingly generates a first sinusoidal wave signal BOP and a second sinusoidal wave signal BRP. After the first sinusoidal wave signal BOP and the second sinusoidal wave signal BRP are transmitted to the comparator RP1 and RP2, the comparator RP1 and RP2 respectively outputs hall signals HC1 and HC2 according to a comparison operation. Next, the logic circuit 112 receives a pulse modulation signal PW generated from the external PWM generator 120 and the hall signals HC1 and HC2 so as to respectively output the switch signal H1, H2, L1 and L2 for controlling the switched-on or switched-off state of each switch unit (not shown) in the driving circuit 114. Afterwards, the driving circuit 114 alternatively outputs the first output signal VOUT1 and the second output signal VOUT2 to the single-phase DC motor 130 for making the single-phase DC motor 130 rotate.
However, when the single-phase DC motor 130 rotates, a Back Electro-Motive Force (BEMF) must be generated in the single-phase DC motor 130, wherein polarity of the BEMF is opposite to that of the applied voltage. Therefore, a current flowing through the single-phase DC motor 130 may be affected by the BEMF, so that a current flowing through the single-phase DC motor 130 may generate change correspondingly, for example the waveform of saddle-type current shown in FIG. 2. Accordingly, in the phase-changing duration of outputting the first output signal VOUT1 and the second output signal VOUT2 (that is the voltage level of the hall signals HC1 and HC2 are low voltage level), the current flowing through the single-phase DC motor 130 may change sharply (for example higher tail current value of the saddle-type current). The sharply changing current will affect operation of the single-phase DC motor 130, so as to generate mechanical noise when the single-phase DC motor 130 rotates.